1. Field of the Invention
The present invention relates to a device for the plasma treatment of gases, in which the gas to be treated is excited by the plasma.
2. Description of Related Art
Devices for the plasma, especially microwave-sustained plasma, treatment of gases of this type generally comprise a hollow structure forming a waveguide intended to be connected to a microwave generator and means for making the gas to be treated flow through the said structure in a region in which the amplitude of the electric field associated with the incident wave is high, and preferably a maximum.
The principle of these gas treatment techniques consists in creating, by means of an electrical discharge in a majority gas or plasma gas (argon, nitrogen, krypton, xenon, etc.) containing impurities or gaseous effluents, a plasma within which the initial molecules are excited, ionized and dissociated into smaller fragments, atoms and radicals. Next, these species recombine and react together and/or with auxiliary gases to give new species which may be removed from the gas by conventional methods such as, for example, reactive adsorption. The molecules of the basic gas to be purified or decontaminated are, when they leave the plant, unchanged and in their ground state. For further details about the various devices enabling excited plasmas to be created, reference may be made for further details to the work entitled “Microwave Excited Plasmas” by M. Moisan and J. Pelletier, Elsevier, 1992.
The document U.S. Pat. No. 5,750,823 describes a device for the plasma treatment of a gas, which is used to excite the gas by sustaining in it an electrical discharge by means of an electric field associated with incident electromagnetic waves. The latter are excited by a field applicator which is formed by the hollow waveguide structure and is itself supplied with microwave power from a conventional waveguide, also formed by the hollow structure.
If the plasma is sustained by an electromagnetic surface wave, the field applicators used are of the surfatron-guide or surfa-guide type.
The means used for making the gas flow consist of a tube made of dielectric material in which the discharge is created.
Further examples of such gas treatment systems are described in the documents DE-A-4 428 418 and U.S. Pat. No. 5,300,202. These systems are operated at low pressure. The document EP-A 0 295 083 describes a device for the plasma treatment of gases, comprising a resonance cavity operating at atmospheric pressure.
These gas treatment devices, although advantageous in terms of conversion yield, energy efficiency and low operating sensitivity to variable operating conditions, have a major drawback in so far as they are ill-suited or not very suitable for the treatment of gases whose decomposition gives an appreciable amount of solid materials, generally in powder form.
This is because these materials may block the tube through which the gas to be treated flows and in which the discharge is created, or, at the very least, they may modify its initially optimized internal geometry, interfere with heat exchange and impede the cooling of the tube, or may even impair the dielectric properties of the latter and disturb the propagation of the surface wave.
To alleviate this drawback, it is possible to use a gas treatment device in which the gas is converted into a plasma, not in a confined tubular space, but in a relatively open volume spatially separated from the waveguide. In this case, one of the possible arrangements is that of a plasma torch supported and supplied by the waveguide. A torch suffers from none of the aforementioned drawbacks caused by the potential formation of solid deposits, in so far as the plasma does not form in the gas feed line for the torch but downstream of the latter, at the end of the torch nozzle.
To produce a gas treatment device, the free end of the torch, at which the plasma forms, emerges in a chamber which has the main function of collecting, in a sealed manner, the gases after treatment in the plasma and to receive the solid deposits produced by the treatment. The gases thus collected are then sent to a discharge line. Thereafter, they can be used as they are, or can undergo a post-treatment, or else can be discharged into the atmosphere.
A known example of such a microwave plasma torch is the “axial injection torch” (AIT). Such a torch is described, for example, in the article “An atmospheric pressure waveguide-fed microwave plasma torch” (Plasma Sources Sci. Technol. 3 (1994) pages 584 to 592) or in Certificate of Addition Application FR-A-2 533 397. The AIT torch has been applied to the destruction of SF8 and C2F6. However, its architecture is relatively sophisticated and its construction is quite complex and expensive. In addition, it is intrinsically difficult to provide the gas circuit with very good sealing. This is a very serious drawback in the case of the treatment of gases containing impurities or effluents whose conversion gives dangerous gases (for example, corrosive halogenated compounds) or else gases of high added value (krypton and xenon). Finally, the length of the feed channel for the AIT nozzle is quite long. This may pose impurity recondensation problems.
It is an object of the invention to alleviate these drawbacks.